Polishing crystal glass



United States Patent 3,546,037 POLISI HNG CRYSTAL GLASS Erich Siilzle,134a Georgenstrasse, 8 Munich 13, Germany No Drawing.Continuation-impart of application Ser. No. 322,048, Mar. 26, 1963, nowPatent No. 3,290,193. This application May 24, 1966, Ser. No. 552,425Claims priority, application Germany, May 26, 1965, S 97,315 Int. Cl.C030 15/02 US. Cl. 156-24 8 Claims ABSTRACT OF THE DISCLOSURE Glassarticles are polished by being contacted with a bath containing sulfuricacid of at least 63% concentration and also containing hydrofluoric acidin which the concentration of fluorosilicic acid formed in the bath ismaintained at below saturation.

This application is a continuation-in-part of Ser. No. 322,048, filedMar. 26, 2963, now Pat. No. 3,290,193.

The present invention relates to a method of polishing glass, and moreparticularly to the polishing of crystal glasses.

In the known processes of polishing pressed or blown glass objects, thesame are immersed for a short time in a polishing bath consisting ofdilute sulfuric acid containing hydrofluoric acid, then taken out of thepolishing bath and the salt layer formed on the surface of the glass isremoved in a rinsing bath. This alternate treatment is repeated severaltimes until the desired surface brilliance is achieved.

The rinsing bath in former times was water. However, this had thedisadvantage that as a result of the repeated immersions a large amountof water passes into the polishingbath so that after a short time thepolishing bath has to be partly regenerated. This provides an additionalproblem in that acid waste liquors of such concentrations, particularlythose containing substantial amounts of hydrofluoric acid, may not bedischarged by being passed into public streams.

To overcome these difii'culties, a process was proposed in which therinsing bath used was concentrated sulfuric acid of such highconcentration that it acts as a dehydrating agent. In this way, the saltlayer on the surface of the glass resulting from the polishing bath islikewise removed and all that then remains to be done on attaining thedesired brilliance after a suflicient number of washings is to rinse thecompletely polished object once with water. This procedure in generalobviated the problems of the exhausted acid.

Another improvement in the above described process was the addition ofheavy metal salts, such as iron chloride, iron sulfate, zinc sulfate,copper sulfate, silver sulfate or also potassium permanganate or chromicacid to the polishing bath for the purpose of reducing the number ofalternate treatments.

All of the known processes were in general carried out using in thepolishing bath a concentration of sulfuric acid which did not exceed60%. A report has been published on scientific investigations into theproperties of polishing baths with various concentrations of sulfuricacid and hydrofluoric acid, which shows that a sulfuric ICC acidconcentration of about 60%, particularly a sulfuric acid concentrationabove 63%, gives rise to serious flaws in the glasses. It was found thatwhen the concentration of sulfuric acid in the polishing bath reachesthis high level, precipitations of fluorosilicic acid occur, and thisacid forms spots or bulges on the glass which are extremely difficult toremove. These precipitations of fluorosilicic acid on the glass surfacesare due to an oversaturation of the bath, and this oversaturationprevents decomposition of the acid to silicon tetrafluoride which can nolonger escape from the bath when such saturation level is reached.

It is accordingly a primary object of the present invention to provide anew method of polishing glass articles, and more particularly crystalglass articles, which avoids all of the enumerated disadvantages of theknown methods and which provides for the possibility of achieving thedesired brilliancy by a single immersion of the article in the polishingbath.

It is yet another object of the present invention to provide a simpleand direct method of achieving a desired brilliancy on a glass articleby immersion of the same in a polishing bath, using only a singleimmersion wherein improvements are also obtained in connection with thesubsequent rinsing of the polished glass article.

Other objects and advantages of the present invention will be apparentfrom a further reading of the specification and of the appended claims.

With the above and other objects in view, the present invention mainlycomprises a method of polishing crystal glass, which comprisescontacting the glass to be polished with a bath containing sulfuric acidof at least 63% concentration and also containing hydrofluoric acid,while maintaining the concentration of fluorosilicic acid which isformed in the bath below the saturation level of said fluorosilicicacid.

Thus, the present invention provides for the polishing of crystal glassarticles, especially glass containing less than 10% of lead, whichincludes highly leaded pressed, pre-pressed or thick walled handblownglasses in a polishing bath containing both sulfuric acid andhydrofluoric acid, wherein the sulfuric acid concentration is maintainedat at least 63%, and the amount of fluorosilicic acid in the bath iskept below the saturation level.

It is a particular advantage of the present invention that it ispossible to achieve the desired brilliancy by a single immersion of thearticle to be polished in the polishing bath, and as a result, it is notnecessary to carry out repeated alternate treatments in a polishing bathfollowed by a water bath or a sulfuric acid bath. Consequently, themethod of the present invention provides for the possibility of avoidingcostly apparatus and also provides for a reduction in the number ofpersonnel required for the transfer of the baskets loaded with the glassarticles from one bath to the other.

By the immersion of the articles to be polished in the polishing bathfor a period of from a few minutes up to about 20 minutes, without anyalternate treatment, excellent surface quality and particularly handsomebrilliancy are achieved Without formation of any waste liquids ordilutions of baths. Due to the particularly high sulfuric acidconcentration which is used by the process of the present invention, theglass-covering layer formed on the glass surface by the polishingreaction is immediately dissolved and the polishing reaction may then becontinued for the required time without any obstruction of a saltcoating.

I have found that as soon as the saturation of fluorosilicic acid isreached, precipitations of fluorosilicic acid on the glass surface occurand this causes the marring of the articles and consequent rejectionthereof. Therefore, in accordance with the present invention, theconcentration of fluorosilicic acid in the polishing bath must bemaintained at below the saturation level thereof by suitable means, forexample by continuously withdrawing part of the liquor and replenishingthe withdrawn part with fresh polishing acid.

The saturation concentration of fluorosilicic acid (HgsiFg) depends to aslight extent on the sulfuric acid concentration of the bath which,according to the present invention, is at least 63%, and is generallybetween about 63% and 75 In general, the permissible degree ofconcentration of fluorosilicic acid in the bath is from 2.5% to 4% of HSiF however, it is advantageous to maintain the range at between about3% and 3.8%.

In practice, when the concentration of the fluorosilicic acid approachesthe above mentioned level, there appear on the surface of some of thearticles to be polished, small deposits, at first detached, which arenot as yet harmful. Since in practice the polishing operation isgenerally carried out in vats of substantial capacity, and since theconcentrations of the individual ingredients of the bath are notidentical in all areas, there may already occur within localized zonesin the bath spots of saturation before the bath as a whole exhibits aninadmissably high concentration of fluorosilicic acid. As soon as thefirst signs of deposition of the fluorosilicic acid occur, it is in mostcases sufficient, as the first expedient, to stir the bath vigorouslyand then to resort to dilution of the bath for the continued performanceof the polishing operation. It is not necessary to interrupt thepolishing operation at the first signs of increasing concentrationsince, as indicated above, an undesirably high concentration of H SiFhas not as yet occurred in the bath as a whole.

The concentration of fluorosilicic acid in the bath can be checked atany time :by periodically taking specimens and analyzing the same. Forthis purpose, a method of volumetric analysis is available which permitsthe determination of the amount of H SiF present in liquids containinghydrofluoric and sulfuric acid (Method of N. S. Nikolajev; note Gmelin,Silicon part B, page 196).

The concentration of hydrofluoric acid in the polishing bath of thepresent invention is maintained within the normal limits, namely fromabout 3% to 10%. Likewise,

the temperature used for the polishing is also maintained within normallimits, e.g. between about 35 C. and 70 C. To prevent an accumulation ofsalts resulting from the polishing reaction in the polishing bath, thesalt deposits may be removed in any of the customary manners, forexample by filtration, cooling and sedimentation. The removal of saltcollecting on the bottom may be brought about, for example, at the sametime when the bath is being replenished for the purpose of maintainingthe concentration of fluorosilicic acid within the bath below thesaturation level.

To maintain the desired concentration of hydrofluoric acid in thepolishing bath it is advantageous to introduce the hydrofluoric acidcontinuously near the bottom of the bath, for example, by means ofdistributor pipes. Particularly good results have been obtained byintroducing hydrogen fluoride in gaseous condition.

The following examples are given to further illustrate the presentinvention. The scope of the invention is not, however, meant to belimited to the specific details of the examples.

EXAMPLE 1 The articles to be polished are made of thick walled, heavy,hand blown lead crystal. The glass contains 24% of PhD, 59% of SiO 0.1%of BaO, 12% of K 0, 3% of Na O and 1.8% of B 0 The articles are immersedin a polishing bath consisting of sulfuric acid of 65% concentration,containing 5% of hydrofluoric acid, at C. The articles are left in thepolishing bath for 10 minutes, while being occasionally moved about init, and are then taken out and all the acid rinsed off with pure water.

The polishing effect obtained in this manner is perfect. No depositionof fluorosilicic acid appears on the glass surfaces.

EXAMPLE 2 The articles to be polished are thick-walled, pressed leadcrystal articles containing 19% of PbO, 63.5% of SiO 12.5% of K 0, 3.5%of Na O and 1.5% of B 0 The procedure used in Example 1 is repeated withthe exception that the sulfuric acid concentration is maintained at 72%,the hydrofluoric acid concentration at 7%, and the temperature of C. isused. The time the articles remain in the polishing bath is 8 minutes,after which the acid is rinsed off with water of 50 C. The resultingbrilliance and surface quality are outstanding.

EXAMPLE 3 The articles to be polished are made from potash glasscontaining 3% of PhD, 73% of SiO 3.5% of BaO, 12% of K 0, 4% of Na O and4.5% of B 0 The glass articles were previously ground with diamonddiscs. The polishing bath used contains 63% of sulfuric acid and 5% ofhydrofluoric acid and is heated to 55 C. After a polishing time of 12minutes, the desired degree of brilliance is achieved. The glassarticles are taken out of the polishing bath and the acid rinsed offwith water at 50 C.

EXAMPLE 4 The glass articles to be polished are made from thinwalled,lead-free soda glass containing 75% of SiO 4% of BaO, 4.5% of K 0, 12%of Na O and 4.5% of B 0 The glasses were first ground with naturalstones.

The polishing bath contains 66% of sulfuric acid and 4% of hydrofluoricacid and is heated to 50 C. The polishing time is 8 minutes. After thistime, the desired degree of brilliance is attained, the glass articlesare taken out of the polishing bath, and the acid rinsed off with waterat 50 C.

To renew the hydrofluoric acid in the polishing bath during thepolishing operation hydrogen fluoride gas is introduced into the bath.

Although the invention has been described in particular with respect tothe polishing treatment of particular glasses, it is to be understoodthat variations and modifications of the invention can be made withoutdeparting from the spirit or scope of the invention. Such variations andmodifications are accordingly meant to be comprehended within themeaning and scope of equivalents of the appended claims.

I claim:

1. The method of polishing a glass article, which comprises contactingsaid glass article with an aqueous polishing bath containing 63% toabout 75% by weight of sulfuric acid and also containing about 3% toabout 10% by weight of hydrofluoric acid, both based on the total weightof the bath, while maintaining the concentration of formed fluorosilicicacid in the bath at below saturation level and not blowing air throughthe bath to carry gaseous hydrogen fluoride to the glass article,removing the glass article from the polishing bath and then washing theglass article with water.

2. Method according to claim 1 wherein the content of hydrofluoric acidin the bath is renewed by introducing hydrogen fluoride gas therein.

3. Method according to claim 1, wherein the concentration offluorosilicic acid in the bath is maintained at between about 2.5% and4% by weight, based on the total weight of the bath.

4. Method according to claim 1, wherein the concentration offluorosilicic acid in the bath is maintained between about 3% and 3.8%by weight, based on the References Cited total weight of the bath. E AENTS 5. Method according to claim 1 wherein the tempera- UNITED STAT S PT b t 2,999,013 9/1961 Meth 1566 2 ggl gf bath mamtamed between a3,290,193 12/1966 Salzle 15614 6. Method according to claim 1 whereinthe glass 5 1,777,321 10/1930 Meth is lead crystal glass.

7. Method according to claim 1, in which said contact- JACOB STEINBERGPnmaly Examiner ing with polishing bath is performed in only a single USCl XR stage. w'

8. Method according to claim 7, in which said washing 10 is performed inonly a single stage.

